A method for producing graft copolymers of a natural polymer, selected from the group consisting of cellulose and protein fibers, with vinyl monomers, comprising the steps of impregnating the natural polymer selected from the group consisting of cellulose and protein fibers with an aqueous solution of ferrous salt, removing excessive ions of ferrous iron, and grafting vinyl monomers to said natural polymer from an aqueous solution, aqueous emulsion or aqueous dispersion of a vinyl monomer, containing hydrogen peroxide and a reducing agent which is selected from the group consisting of sodium sulfide, glucose, sodium hypophosphite, sodium bitartrate and hydroquinone, and used in an amount of 0.002 to 0.02 percent by mass.